Fuel Tanks Not Equalizing

yup... that's why to took so long to find it.. when they would blow air thru the line it would blow it clear then 1 hr or a day later it would float over and block it again. Turn the truck off and it would float away then block it again later. Took about 3-4 weeks of that before it floated by the fuel cap and it was seen...

 

The system needs a return because the pump is always pulling/ supplying more fuel to the injectors than they can use so it has to flow back to the tank to be cooled and picked up again later.
All you are looking for with the test is making sure fuel is flowing back to the tank through those lines. The fuel will not be under pressure in the return. It will just steadily flow. So you can do it into fuel cans and just pour it back into the tanks.

Also. I agree with the debris possibility randomly blocking lines. That’s why when I get fuel I use a rag to clean off the pump nozzles. There always seems to be mud and crap all over them. Never understood how someone can make such a mess of them.
You can try to look around the inside of the tank through the fill cap hole with a high powered led flashlight and see if you can spot anything floating around down there. But stuff like that always hides out of site.